Dr Barry Brook of the University of Adelaide, and colleagues, question suggestions in recent scientific papers that human activity is pushing planetary ecology past "tipping points", altering terrestrial life rapidly and possibly irreversibly.

Some scientists, for example, argue the number of species lost has already crossed a threshold, compromising ecological function.

Brook says while ecological tipping points apply in some cases, such as the Amazon, he does not believe humans can have the sort of global-scale impact on the terrestrial biosphere, that meteorites or exploding volcanoes are capable of.

In the past, such catastrophic events drove large and rapid planetary-scale changes that shifted Earth's terrestrial ecology into a completely new state, he says.

By contrast, says Brook, human activity has a much more local and piecemeal effect on terrestrial environments and this does not add up to one focused global driver of change.

In his study, he looked at the effect of human activity on species loss, habitat fragmentation and land use.

"Things happen in a patchy way at different rates in different environments and with different consequences," says Brook.

"That lack of co-ordination suggests relatively smooth changes at the global scale will occur rather than there being a sudden tipping point."

Climate change

The new study also looked at climate change.

Brook says even though climate change is a result of a global driver (CO2), its impacts vary from region to region and some areas will be more vulnerable than others to the same degree of warming.

"The argument that climate change will cause a [global] tipping point in terrestrial ecosystems is weak," he says.

Brook emphasises he does not dispute ecological tipping points apply in some cases.

For example, he says ocean acidification may be reaching a tipping point, and tipping points may be relevant in some regions, for example the Amazon.

"What we're saying is that at a planetary scale there are no mechanisms that can explain how the whole earth system can shift from one state to another."

Brook believes the tipping point concept has strong psychological effects on humans.

He says before a tipping point is reached, people can delay taking action, and after it has passed, people feel it's too late to take action.

"On one side of the tipping point you're complacent, on the other side you're fatalistic," says Brook.

"If there is no strong tipping point then it's never too late or too early to intervene."

Interconnections

Atmospheric scientist Professor Andrew Pitman from the University of New South Wales agrees with Brook and colleagues' analysis of the relatively limited impact that patchy human activity can have on habitat and species across the globe.

"Human land use change doesn't have an impact on triggering tipping points at the global scale," he says.

But Pitman says Brook has failed to consider the role of the atmosphere in driving global-scale change.

"He takes one piece of the jigsaw, the terrestrial biosphere, and talks about it in isolation," says Pitman.

Pitman says while a slow upward trend in CO2 is unlikely to directly trigger a planetary-scale collapse of the terrestrial biosphere, the extreme climatic events linked to climate change are more of a problem.

For example, he says, drought together with burning of the Amazon could lead to major changes in the Earth's atmosphere through release of CO2, aerosols and changes to the global energy and water cycles.

"Water that falls on the edge of the Amazon, falls again three or four or five times as it heads west across the Amazon basin. It recycles its own rainfall and creates its own climate, which interacts with the global climate," says Pitman.

He says a tipping point in a regional area like the Amazon could trigger a global tipping point through the mediating role of the atmosphere.

And just like a collapse of the large Chinese economy would have devastating effects on the global economy, a collapse of the vast Amazon would have devastating effects on the globe's ecology, including biodiversity, says Pitman.

"Clearly the loss of the Amazon would not have as big an impact as a three-kilometre meteorite hitting Western Europe," he says.

"But we don't have much we can do about a three-kilometre meteorite. There is something we can do about CO2 emissions and the consequential impact on the Amazon."